In recent years, braking force has been increased due to the spread of the disc brake. Due to this, sometimes uncomfortable low noise is caused, especially during traveling, due to vibration of the brake rotor sandwiched by the brake pads. Such a phenomenon, called “brake judder”, has been noticed in accompanying with tendency of development of vehicles having high performance and lesser noise. Thus, it is a new technical object to analyze and improve the brake judder problem.
Although the mechanism of brake judder has not been clearly solved, it is believed that the accuracy of the surface run-out of the brake rotor contacting with the brake pads is one cause of brake judder. The accuracy of the surface run-out of the brake rotor is influenced not only by the run-out accuracy of the brake rotor itself, but by the accuracy of the surface run-out of the wheel mounting flange, the axial run-out of the rolling bearings, the accuracy of the raceway surfaces, and the assembling accuracy of the rolling bearings, etc.
It is desirable for the vehicle wheel bearing apparatus, on one hand, to reduce its size and weight to reduce its manufacturing cost and fuel consumption in view of the material savings. On the other hand, it is desirable to increase the wheel bearing apparatus rigidity to improve steering stability. The desire to reduce the vehicle weight has increased especially in the field of light weight vehicles, such as light weight 4-wheel vehicles, or small size vehicles. The desire to reduce the vehicle weight has increased.
FIG. 13 shows one example of a vehicle wheel bearing apparatus of the prior art intended the weight reduction. This wheel bearing apparatus 50 is a representative structure used for a driving wheel side. The apparatus is formed as a unit including a wheel hub 51 and a double row rolling bearing outer member 52.
The wheel hub 51 is made by forging and has a hollow structure. The wheel hub 51 is integrally formed with a wheel mounting flange 53 on which a wheel (not shown) is mounted. One inner raceway surface 51a is arranged opposite to one 57a of the double row outer raceway surfaces 57a, 57a of the outer member body 57. A cylindrical portion 51b axially extends from the inner raceway surface 51a. The inner circumference of the cylindrical portion 51b is formed with a spline 54 for torque transmission purposes.
An inner ring 55 is press fit onto the cylindrical portion 51b of the wheel hub 51. The inner ring 55 is formed, on its outer circumference, with the other inner raceway surface 55a of the inner raceway surfaces 51a, 55a. An end of the cylindrical portion 51b is plastically deformed radially outward to form a caulking portion 56 to axially secure the inner ring 55 relative to the wheel hub 51.
A double row rolling bearing outer member 52 is integrally formed with a body mounting flange 57b. The flange 57b forms part of a suspension apparatus 58. The inner circumference of the outer member 52 is formed with double row outer raceway surfaces 57a, 57a. The outer raceway surfaces 57a, 57a are arranged opposite to the double row inner raceway surfaces 51a, 55a. As can be appreciated, the double row rolling bearing 52 includes the wheel hub 51, the inner ring 55, and double row balls 59, 59, rollably contained between the inner and outer raceway surfaces 51a, 55a and 57a, 57a. 
Hub bolts 53a are secured at four positions equidistantly arranged along the circumference on the wheel mounting flange 53 of the wheel hub 51. A brake rotor 60 and a wheel are secured on the wheel mounting flange by fastening nuts onto the hub bolts 53a. As shown in FIG. 14, the wheel mounting flange 53 of the wheel hub 51 is formed with bolt apertures 61 through which the hub bolts 53a are secured. Cut-out portions 62, each having a circular arc configuration, are formed on the outer circumference of the wheel mounting flange 53 between the bolt apertures 61 by avoiding the vicinity of each bolt aperture 61.
The cut-out portions 62 are formed so that a deepest portion 62a of each cut-out portion 62 is positioned radially inward of a pitch circle diameter A of the bolt apertures 61 and close to a line B connecting the centers of adjacent bolt apertures 61. In addition, the wheel mounting flange 53 is formed so that it has a thin thickness in a radially outward region 63 including the bolt apertures 61. The wheel mounting flange 53 has a normal thickness in a radially inward region 64, including the bolt apertures 61, so as to have sufficient bending rigidity. This prevents reduction of the bending rigidity of the wheel mounting flange 53 while attempting to reduce its weight (see e.g. Patent Document 1 mentioned below).
According to the prior art wheel bearing apparatus 50, both the prevention in the reduction of the bending rigidity of the wheel mounting flange 53 and achievement of a reduction of weight can be obtained by cutting out portions between of the hub bolts 53a. Thus, the wheel mounting flange 53 has a flower petal-like configuration and keeps the thickness of the radially inward region of the wheel mounting flange 53 normal. In addition, a wheel bearing apparatus 70 shown in FIGS. 15 and 16 is known to further reduce its weight. This wheel bearing apparatus 70 is used for various light weight vehicles such as light weight 4-wheel vehicle or small sized cars. The apparatus 70 includes an inner member 73 with a wheel hub 71 and an inner ring 72, an outer member 74, and double row balls 75, 75 rollably contained between the inner and outer members 73, 74.
The wheel hub 71 has an integrally formed wheel mounting flange 76 on one end. The wheel hub 71 is formed on its outer circumference with an inner raceway surface 71a. A cylindrical portion 71b axially extends from the inner raceway surface 71a. An inner ring 72 is press fit onto the cylindrical portion 71b. The inner ring outer circumference includes another inner raceway surface 72a. The inner ring 72 is axially secured by a caulked portion 71c. The caulked portion 71c is formed by plastically deforming the end of the cylindrical portion 71b radially outward.
Bolt apertures 76a are equidistantly formed along the outer circumference of the wheel mounting flange 76. Hub bolts 76b are press fit therein to fasten a brake rotor R and a wheel W.
The outer member 74 is formed with double row outer raceway surfaces 74a, 74a on its inner circumference. The outer raceway surfaces 74a, 74a are arranged oppositely to the double row inner raceway surfaces 71a, 72a. Double row balls 75, 75 are rollably contained between the outer and inner raceway surfaces 74a, 74a and 71a, 72a, via cages 77. Seals 78, 79 are mounted on both ends of the outer member 74. The seals 78, 79 seal annular openings formed between the outer and inner members 74 and 73.
As shown in FIG. 16, the wheel mounting flange 76 of the wheel hub 71 includes a plurality of radially extending partial flanges 80. Each flange 80 has a bolt aperture 76a at its tip end. A plurality of ridge-shaped reinforcing ribs 80a are formed on the inner surface of the wheel mounting flange 76, which is the opposite side of where a wheel is mounted.
The wheel mounting flange 76 is formed by a plurality of radially extending partial flanges 80. Each flange 80 has a bolt aperture 76 at its tip end. A plurality of ridge-shaped reinforcing ribs 80a are formed on the partial flanges 80. Thus, it is possible to reduce the weight of the wheel hub 71 and to improve the durability of the wheel hub 71 due to a reduction of stress caused in the wheel mounting flange as compared with a wheel hub provided with a circumferentially continuous wheel mounting flange (see e.g. Patent Document 2 mentioned below). Patent Document 1: Japanese Laid-open Patent Publication No. 94905/2003. Patent Document 2: Japanese Laid-open Patent Publication No. 119505/2005
As described above, the wheel bearing apparatus 50, 70 are intended to be used in small passenger car including light weight 4-wheel vehicle or small cars. The run-out of the wheel mounting flange 53, 76 is at most 100 μm or less when used in small cars even thought the wheel mounting flanges 53, 76 of the wheel hub 51, 71 are formed with a flower petal-like configuration or partial flanges 80. Accordingly, particular problems are not caused when the wheel hubs 51, 71 are used in small cars. However, requirement for reduction of NHV (noise vibration harshness) of a vehicle has been increased when they are used in middle or larger class passenger cars. Especially when they are used for wheel bearing apparatus for a front wheel, a highly accurate run-out of the wheel mounting flanges 53, 76 of 30 μm or less would sometimes be required.
Accordingly it has been required to have a flange configuration that can suppress the run-out of the wheel mounting flanges 53, 76 and reduce their weight without the influence of heat treatment applied to the wheel hubs 51, 71. In addition, the wheel mounting flanges 53, 76 are liable to bend when a bending moment is applied. Thus, the brake rotors 60, R will also bend. Although it is necessary to suppress the bending of the wheel mounting flange, the amount of bending of the wheel mounting flanges 53, 76 and accordingly the brake rotors 60, R would be larger than that of wheel mounting flanges having normal disc shaped configuration. This is due to the fact that regions between the hub bolts 53a, 76b are cut out and thus there is no material forming the continuous wheel mounting flange.